The mechanism of renal excretion of ceftizoxime was investigated by renal clearance test and stop flow analysis in rabbits and dogs. The serum half life of ceftizoxime after dosing of probenecid and ceftizoxime in combination was about twice longer in rabbits than and almost the same in dogs as that after dosing of ceftizoxime alone. Renal clearance test and stop flow analysis were performed with the following results: (1) Renal clearance of ceftizoxime was about 3.2 times higher in rabbits than and almost the same in dogs as creatinine clearance. (2) Ceftizoxime was scarcely excreted and reabsorbed in the proximal and distal areas of dogs. (3) Ceftizoxime was excreted in the proximal area and not reabsorbed in the distal area of rabbit. These findings indicate that 60% of the urinary excretion of ceftizoxime was obtained through proximal tubule and remaining amount by glomerular filtration in rabbits, and most of the urinary excretion was obtained by glomerular filtration in dogs.